The present invention relates to a rocker arm for opening and closing a valve which is tilted by a cam and provided on a cylinder head and a fabricating method thereof. More specifically, the present invention relates to the rocker arm having a body made of sheet metal and a fabricating method of the body.
A rocker arm formed of sheet metal is disclosed in Japanese Patent Application Laid-Open No. 4-259611 (1992). This rocker arm is of an end pivot type which is tilted by a cam, with one end in the longitudinal direction being a supporting point, to open and close a valve. Moreover, a roller is mounted in a body of the rocker arm. The body has a pair of opposed side walls and a connecting wall which connects the side walls. These walls are formed by bending one metal plate into a U-shape. A valve fitting section is provided in one end area in the longitudinal direction of the connecting wall, and a pivot receiver is provided in the other end area in the longitudinal direction of the connecting wall. An upper end of the valve is fitted into the valve fitting section, and an upper end of a pivot section provided on a cylinder head is fitted into the pivot receiver.
The roller is supported rotatably between the pair of side walls via a supporting shaft in a state that a part of the roller protrudes from a through hole provided in a middle portion of the longitudinal direction in the connecting wall.
In this rocker arm, the valve fitting section of the body is formed in the following manner. The body is bent so as to have a substantially U-shaped section, and the one connecting wall of the body is pushed up to a halfway position in the up-and-down direction of the pair of side walls so as to be bent into a reverse M-shape as viewed from an end surface.
In the prior example, the valve fitting section of the body is obtained by forcibly bending the body into the reverse M shape. For this reason, as shown in FIGS. 8A through 8C, for example, crazing, which causes lowering of strength, and a sink mark, which causes lowering of shape accuracy, easily occur on an outer surface of the bent section of the pair of side walls 51 and 52 and the connecting wall 53. Additionally, the sink mark varies a width dimension W of the valve fitting section 50 which results in increases in a generation rate of defective products.
Therefore, it is an object of the present invention to provide a rocker arm which has a form such that a valve fitting section formed on a body made of sheet metal can be processed without generation of defects such as crazing and sink marks and relates to a method of fabricating the body of the rocker arm.
The other objects features and advantages of the invention will become clear from the following text read in conjunction with the drawings.
A rocker arm of the present invention, for opening and closing a valve when tilted by a cam, which is provided on a cylinder head has a body made of sheet metal which is obtained by bending one metal plate into a substantially U shape so as to form a pair of opposed side walls and connecting walls for connecting the side walls. A convex valve fitting section, which has a convex lower side for contacting the valve stem and an opposing concave upper side, is formed in one of the connecting walls of the body by press working including a process for forming a non-uniform cross section due to plastic flow.
According to the rocker arm of the present invention, the valve fitting section of the body is obtained not by the bending process but by press working including the non-uniform cross section processing. For this reason, crazing and sink mark do not occur, and a width dimension of the valve fitting section can be managed accurately.
The rocker arm of the present invention is preferably constituted so that the connecting walls have an surrounding area adjacent where the valve fitting section will be formed which is a circumferential section, and a thickness of the area where the valve fitting section will be formed is increased by the non-uniform cross section processing for plastically flowing material of the circumferential section to the area where the valve fitting section will be formed, and the valve fitting section is then formed in the thickened area.
The rocker arm of the present invention is preferably constituted so that the connecting wall has a pair of protruded walls, and the protruded walls are protruded by the non-uniform cross section process for plastically flowing the sections of the side walls, and between the protruded sections is the convex valve fitting section.
A method of fabricating a rocker arm body according to the present invention, the body being made of sheet metal and being provided with a pair of opposed side walls and connecting walls for connecting the side walls, the rocker arm opening and closing a valve when tilted by a cam and provided on a cylinder head, includes the first step of obtaining a developed form of the body from one metal plate; and the second step of forming a convex valve fitting section on one of the connecting walls of the body by means of press working including a non-uniform cross section process inducing plastic flow.
According to the fabricating method of the present invention, since the valve fitting section of the body is formed by press working including the non-uniform cross section process, crazing and a sink mark do not occur. For this reason, the rocker arm body can be fabricated having a width dimension of the valve fitting section that can be managed accurately.
In the method of fabricating the rocker arm body according to the present invention, the second step plastically flows sections of the side walls on both sides of the connecting walls in an area where a valve fitting section will be formed to the area where the valve fitting section will be formed so as to increase a thickness of the area where the valve fitting section will be formed, and sinks the thickened areas where the valve fitting section will be formed so as to form the convex valve fitting section.
In the method of fabricating the rocker arm body according to the present invention, the second step plastically flows at least sections of the side walls of the body so as to form a pair of opposed protruded walls, and between the protruded walls is the convex valve fitting section.